Method and apparatus for coating electroluminescent phospors

ABSTRACT

A process for preparing particles of zinc sulfide-based electroluminescent phosphor having a moisture resistant coating thereon which comprises the steps of selecting a reaction vessel having a given height and a porous disc at the bottom thereof; charging the reaction vessel with phosphor particles and fluidizing the particles by introducing an inert gas into the vessel through the porous disc; heating the reaction vessel to a reaction temperature; introducing a coating precursor into the reaction vessel at a position adjacent the bottom of the vessel but above the disc; introducing a co-reactant into the reaction vessel at a position substantially mid-way of the given height; and maintaining the inert gas flow, the precursor flow and the co-reactant flow for a time sufficient for a reaction to occur and coat the phosphor with the moisture resistant coating. Apparatus for carrying out the process is also disclosed.

[0001] This application claims priority from Provisional PatentApplication No. 60/256,194, filed Dec. 18, 2000.

TECHNICAL FIELD

[0002] This invention relates to zinc sulfide-based electroluminescentphosphors and more particularly to a method and apparatus for coatingsuch phosphors with a moisture inhibiting coating.

[0003] Still more particularly, it relates to a method and apparatus formaking such phosphors in commercial quantities.

BACKGROUND ART

[0004] It is known from U.S. Pat. No. 6,064,150 that a coating ofaluminum nitride or aluminum nitride amine on an electroluminescentphosphor particle allows the manufacture of electroluminescent lightsources having good life characteristics. It is also known to coat suchphosphors with a metal oxide hydroxide.

[0005] It would be an advance in the art if such coatings could beapplied to large batches of phosphor materials.

DISCLOSURE OF INVENTION

[0006] It is, therefore, an object of this invention to obviate thedisadvantages of the prior art.

[0007] It is another object of the invention to enhance the manufactureof commercial quantities of coated electroluminescent phosphors.

[0008] These objects are accomplished, in one aspect of the invention,by a process for preparing particles of zinc sulfide-basedelectroluminescent phosphor having a moisture resistant coating thereonwhich comprises the steps of selecting a reaction vessel having a givenheight and a porous disc at the bottom thereof; charging the reactionvessel with phosphor particles and fluidizing the particles byintroducing an inert gas into the vessel through the porous disc;heating the reaction vessel to a reaction temperature; introducing acoating precursor into the reaction vessel at a position adjacent thebottom of the vessel but above the disc; introducing a co-reactant intothe reaction vessel at a position substantially mid-way of the givenheight; and maintaining the inert gas flow, the precursor flow and theco-reactant flow for a time sufficient for a reaction to occur and coatthe phosphor with the moisture resistant coating.

[0009] The apparatus for manufacturing commercial quantities of zincsulfide-based electroluminescent phosphor having a moisture resistantcoating thereon, comprises a reaction vessel having a given height and aporous disc at the bottom thereof; a supply of phosphor particles withinthe vessel; a first supply of an inert gas for fluidizing the particles,the first supply of inert gas entering the vessel through the porousdisc; a heater surrounding the reaction vessel for heating the reactionvessel to a reaction temperature; a supply of a coating precursor; firstmeans for conducting the coating precursor from the supply to thereaction vessel, the first means entering the reaction vessel at thebottom of the vessel at a position above the porous disc; a supply of aco-reactant; and a second means for conducting the co-reactant from thesupply to the reaction vessel, the second means entering the reactionvessel at a position substantially mid-way of the given height.

[0010] This method and apparatus allows for the commercial production ofsizeable quantities of coated electroluminescent phosphor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The single FIGURE is a diagrammatic illustration of the apparatusof the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0012] For a better understanding of the present invention, togetherwith other and further objects, advantages and capabilities thereof,reference is made to the following disclosure and appended claims inconjunction with the above-described drawings.

[0013] Referring now to the drawings with greater particularity, thereis shown in the FIGURE a reaction vessel 10 a that is preferably formedof stainless steel having a diameter of greater than 6 inches, a heightof about 6 feet and a capacity of greater than 50 kilograms. In apreferred embodiment, the vessel 10 a has a diameter between 10 and 12inches, a height of 6 feet and a capacity of greater than 150 kilograms.In actuality, capacities of more than 300 kilograms are possible. Thevessel 10 a is surrounded by a suitable heater 30 a that is capable ofbringing the reaction vessel 10 a to coating temperature between 150 and225° C. The coating precursor, which can behexakis(dimethylamido)dialuminum, is fed from a supply 20 a afterentrainment with nitrogen from a supply 22 a thereof through tube 32into the vessel 10 a. The entrance of tube 32 into vessel 10 a is at apoint about 1.0 cm above a porous disc 12 a. The tube 32 has a narrowtip within the vessel 10 a, the tip being located in the center of thereaction vessel.

[0014] The co-reactant, which can be diluted anhydrous ammonia, is fedfrom a supply 24 a into the side of the reaction vessel 10 a, via a tube33, at a point substantially mid-way of the height. Tube 33 has a narrowopening slit that is located in the center of vessel 10 a. Theco-reactant can pass through a Unit mass controller 26 a prior toentering the vessel 1Oa. Dilution can be accomplished by adding purifiednitrogen.

[0015] A supply of inert gas 18 a such as nitrogen is provided forfluidizing the phosphor particles 16. The inert gas is fed from thebottom of the reaction vessel 10 a, through the porous disc 12 a, thusensuring a good fluidization of the particles.

[0016] This process and apparatus solve the problems of the prior arttechnique wherein the precursors, such as the above-citedhexakis(dimethylamido)dialuminum were charged into the reaction vesselthrough the porous disc 12 a. It has been found that the relativelyextreme reactivity of the precursor caused a nitride-plating reaction tooccur within the pores of the disc 12 a, soon plugging the holes in thedisc and stopping the desired reaction. This occurs even more rapidlybecause of the elevated temperatures of the reaction vessel, which arebetween 150 and 225° C.

[0017] By charging the precursor into the reaction vessel above theporous disc the plating problem is eliminated. Also, by feeding thenitride precursor from the side instead of from the top of the reactionvessel, as did the prior art, the precursor is exposed for a shortertime inside the hot vessel. Additionally, this reaction vessel designcan be utilized with other coating processes, such as the reaction oftrimethylaluminum with oxygen and ozone. Feeding an oxygen-ozone mixturefrom the side of the vessel instead of from the top extends the life ofthe very reactive ozone within the vessel allowing greater coatingpotential.

[0018] While there have been shown and described what are at presentconsidered to be the preferred embodiments of the invention, it will beapparent to those skilled in the art that various changes andmodification can be made herein without departing from the scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A process for preparing particles of zincsulfide-based electroluminescent phosphor having a moisture resistantcoating thereon comprising the steps of: selecting a reaction vesselhaving a given height and a porous disc at the bottom thereof; chargingsaid reaction vessel with phosphor particles and fluidizing saidparticles by introducing an inert gas into said vessel through saidporous disc; heating said reaction vessel to a reaction temperature;introducing a coating precursor into said reaction vessel at a positionadjacent said bottom of said vessel but above said disc; introducing aco-reactant into said reaction vessel at a position substantiallymid-way of said given height; and maintaining said inert gas flow, saidprecursor flow and said co-reactant flow for a time sufficient for areaction to occur and coat said phosphor with said moisture resistantcoating.
 2. The method of claim 1 wherein said moisture resistantcoating is aluminum nitride amine.
 3. The method of claim 1 wherein saidcoating precursor is hexakis(dimethylamido)dialuminum.
 4. The method ofclaim 3 wherein said co-reactant is anhydrous ammonia.
 5. The method ofclaim 4 wherein said reaction temperature is about 150 to 225° C.
 6. Anapparatus for manufacturing commercial quantities of zinc sulfide-basedelectroluminescent phosphor having a moisture resistant coating thereon,said apparatus comprising: a reaction vessel having a given height and aporous disc at the bottom thereof; a supply of phosphor particles withinsaid vessel; a first supply of an inert gas for fluidizing saidparticles, said first supply of inert gas entering said vessel throughsaid porous disc; a heater surrounding said reaction vessel for heatingsaid reaction vessel to a reaction temperature; a supply of a coatingprecursor; first means for conducting said coating precursor from saidsupply to said reaction vessel, said first means entering said reactionvessel at said bottom of said vessel at a position above said porousdisc; a supply of a co-reactant; and a second means for conducting saidco-reactant from said supply to said reaction vessel, said second meansentering said reaction vessel at a position substantially mid-way ofsaid given height.
 7. The apparatus of claim 6 wherein said reactionvessel has a diameter of about 10 inches.
 8. The apparatus of claim 7wherein said reaction vessel is stainless steel.